API 650 statment

[Adam001]

In API 650 standard , item 5.2.1 C) , the design internal pressure shall not exceed 18 kpa , and since atmospheric pressure is 100 kpa so does this statement means that internal pressure shall not exceed 118 kpa ? also the external pressure shall not be less than 100.25 kpa ?

 

[mk3223]

Yes, note that one is in the "gage" pressure, and the other the "absolute" pressure.

 

[Adam001]

mk3223 , what do you mean by " one " I don't get it ?

 

[LittleInch]

API 650 does not unfortunately include design pressure as a defined item in section 3.

This is mainly because API 650 is intended for tanks to be at an internal pressure of "approximately atmospheric pressure" see 1.1.1

Appendix F states that if the pressure is less than 1kPa (4 inches of water), then it is classed as "atmospheric" Design pressure higher than that up to 18 kPa needs Appendix F to be applied.

It is clear that the internal design pressure is gauge pressure, i.e. the differential pressure between the internal pressure and local atmospheric pressure. I use the term local to indicate the atmospheric pressure at the location where the tank is as "atmospheric" pressure varies with altitude. You cannot use a design pressure of 118 kPa absolute as if you then build your tank at say 3000m above sea level then your internal pressure will be a lot more than 18 kPa (guage).

Is that clearer?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JStephen]

Atmospheric pressure varies depending on where you are. The pressure and vacuum ratings for tanks and vessels are generally given as gauge pressure, that being the pressure over or under atmospheric pressure.